An electric motor is a machine that converts electrical energy into mechanical energy. Electric motors include DC motors and AC motors. One type of AC motor is an AC induction motor. An AC induction motor is typically driven by 3-phase alternating current provided by an electric motor controller coupled to a 3-phase inverter. The AC motor includes an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, which induces a current in the rotor windings. As the current flows through the rotor windings, a second magnetic field is generated which interacts with the magnetic field from the stator to produce motion. The rotor is attached to the output shaft that is given a torque by the rotating magnetic field. The interaction of the rotor field and the stator field causes rotation of the rotor which can be used to perform work.
Another type of AC motor is a permanent magnet motor (PMM). PMMs have permanent magnets located on the rotor and copper windings located on the stator. The alternating current in the stator windings produces a rotating magnetic field which interacts with the magnetic field from the rotor magnets to produce motion. The frequency at which the stator current oscillates determines the rotor's angular velocity and the resulting angular position.
Electric motors have two mechanical operations, motoring and braking. In a torque vs. speed plot, quadrants I and III of the torque-speed plane represent forward and reverse motoring operations and quadrants II and IV represent forward and reverse braking operations. The braking operation is regenerative when the electric motor is operated as an electric generator such that the kinetic energy of the rotor is converted to electricity and fed back to the power source. Not all operating points in the braking quadrants are regenerative in nature, therefore, regenerative braking is a subset of the braking quadrants. The boundaries of the regenerative braking operation in the braking quadrants need to be determined so that a closed-loop control system does not place an operating point in the non-regenerative braking zone which causes the machine to draw power from the source in order to achieve braking.